The present invention relates to an ultrasonic flow sensor for determining the transit time of an ultrasonic signal, and to a related method.
Ultrasonic flow sensors are used, in particular, to measure the volumetric flow, mass flow, or the flow velocity of a gaseous or liquid medium flowing through a pipeline or the like. A known type of ultrasonic flow sensor includes two ultrasonic converters located such that they are offset in the direction of flow, each of which generates ultrasonic signals and transmits them to the other ultrasonic converter. The ultrasonic signals are received by the other converter and are evaluated electronically. The difference between the transit time of the signal in the direction of flow and the transit time of the signal in the opposite direction is a measure of the flow velocity of the fluid.
FIG. 1 shows a typical design of an ultrasonic flow sensor with two ultrasonic converters A, B, which are diametrically opposed at a distance L from each other. A fluid 1 flows in pipeline 3 with a velocity v in the direction of arrow 2. Measurement path L is tilted relative to flow direction 2 at an angle δ. While a measurement is being carried out, ultrasonic converters A, B send ultrasonic pulses to each other. The signals are decelerated or accelerated, depending on the direction of the flow. The transit times of the ultrasonic signals are a measure of the flow rate to be determined.
FIG. 2 shows a greatly simplified schematic depiction of electrical evaluation circuit 4. The two ultrasonic converters A, B are connected with control and evaluation electronics 4 and are activated by an oscillator with a specified clock frequency 8 (a square-wave signal in this case). Ultrasonic signals 15 generated as a result (only envelope 16 of ultrasonic signals 15 is shown in the figure) travel along measurement path L and are detected by the other ultrasonic converter A, B. Transit time t12 or t21 of signals 15 is measured.
Various methods for determining the transit time of ultrasonic signals are known from the related art. According to a first type of method, the instant when an ultrasonic signal is received is determined unequivocally and exactly. The first zero crossing of the ultrasonic signal that occurs after the signal amplitude has been exceeded a specified threshold value is the “reception time” of the signal. As an alternative, e.g., the instant at which the maximum amplitude occurs, or the instant at which the centroid of the envelope of the ultrasonic signal 15 occurs is used as the “reception time”.
According to a second type of method for measuring transit time, the phase of an ultrasonic signal that has been received is determined with respect to a reference timing signal. It is known, for example, to determine the phase angle (Δφ) of an ultrasonic signal relative to a reference timing signal using a quadrature demodulation scheme, to calculate the total transit time of the ultrasonic signal based on the phase angle (Δφ), and to calculate a remainder (r(t)), which is a whole-number multiple of 2π. The ultrasonic signal is inverted in a segmented manner using a demodulation signal and a phase-shifted demodulation signal. The signals, which have been inverted in a segmented manner, are then preferably filtered or integrated, and the phase angle (Δφ) is determined using a trigonometric calculation. In this evaluation method, the unambiguous range of the calculated phases is equal to the reciprocal of the modulation frequency. The latter is determined by the ultrasonic frequency and/or the properties of the ultrasonic converter, however. This therefore typically results in a relatively small unambiguous range. For signals with a frequency of, e.g., 200 kHz, an unambiguous range of only 5 μs results. Determining the remainder (n*2π) is often a very complex procedure.